1. Field of the invention
The invention relates to an absolute time in pre-groove (hereinafter referred to as ATIP) bit data generator.
2. Description of the Related Art
FIG. 1 shows an ATIP information block of 42 bits. As shown in the drawing, the 42 bit ATIP information block includes a sync pattern of 4 bits, a minute information of 8 bits, a second information of 8 bits, a frame information of 8 bits, and a CRC (cyclic redundancy check) information of 14 bits. The ATIP information provides a correct writing position in a recordable optical disc system.
FIG. 2 shows an block diagram of a conventional ATIP bit data generator, which is disclosed in U.S. Pat. No. 5,506,824, entitled “Frequency modulation to bi-phase data conversion for writable CD ATIP data.” As shown in FIG. 2, the conventional ATIP bit data generator 12 utilizes a bi-phase converter 32 to convert the ATIP frequency modulation (FM) signal into the bi-phase data. Then, a digital PLL (Phase Locked Loop) 38 is used to generate a 2X-frequency clock. Finally, an ATIP decoder 26 is used to generate ATIP bit data according to the bi-phase data using the 2X frequency clock as a reference clock. The bi-phase converter 32 counts the pulse number of a high-frequency clock (generated from the high-frequency PLL 28) in each half period of the ATIP FM signal, and determines the states of the bi-phase data according to the pulse number. That is, when the pulse number is smaller than a threshold value, the bi-phase data is H; and when the pulse number is greater than the threshold value, the bi-phase data is L. The ATIP decoder 26 decodes the ATIP information according to the bi-phase data.
Under normal conditions, the above-mentioned method may generate correct bi-phase data. If the duty cycle of the ATIP FM signal is not uniform, however, the bi-phase converter 32 is influenced and can not generate correct bi-phase data because the pulse number of the high-frequency clock corresponding to each half period is shifted. Furthermore, when the reading speed for the optical disc increases, the output frequency of the high-frequency PLL 28 also correspondingly increases, thereby causing a difficult design, high cost and low resolution.
FIGS. 3a, 3b and 3c show the relationship between the ATIP FM signal and the bi-phase data, wherein FIG. 3a represents the ATIP FM signal, FIG. 3b represents the bi-phase data, and FIG. 3c represents the bi-phase clock signal. When the ATIP FM signal is at high frequency, the bi-phase data is H; and when the ATIP FM signal is at low frequency, the bi-phase data is L.